Technical Field
Embodiments of the invention relate generally to power generation and, more specifically, to a system and method for preheating a heat recovery steam generator.
Discussion of Art
Heat recovery steam generators (“HRSGs”) are devices that recover thermal energy from heat-containing mediums such as flue gas produced via combustion of a fuel, or any other gas and/or plasma that contains recoverable thermal energy. Many HRSGs recover heat from a heat-containing medium by exposing one or more components, typically pressurized/pressure parts such as an evaporator and/or a superheater, to the heat-containing medium. In such HRSGs, the evaporator generates steam by absorbing/recovering some of the thermal energy from the heat-containing medium. The superheater then dries the generated steam by absorbing/recovering more of the thermal energy from the heat-containing medium. The dried steam is then typically distributed to a steam-consuming device, e.g., a steam powered turbine for generating electrical power.
Many power plants, known as combined cycle power generation plants, often use HRSGs to recover thermal energy from a flue gas produced by a primary generator. The recovered thermal energy is then used to power a turbine which in turn powers a secondary generator. Such combined cycle power generation plants typically have two modes of operation, simple-cycle and combined-cycle. During simple-cycle mode, the HRSG is “offline” and not generating steam, and the flue gas produced by the primary generator is typically diverted around the HRSG via a bypass stack such that the pressurized parts are not exposed to the flue gas. During combined-cycle mode, the HRSG is “online,” i.e., the flue gas is directed into the HRSG to generate steam, and the pressurized parts are exposed to the flue gas.
In such power plants, however, it is often the case that pressurized parts of the HRSG, and in particular the evaporator and the superheater, experience thermal stresses during transition from simple-cycle mode to combined-cycle mode due to temperature differences between the pressurized parts and the flue gas. Such thermal stresses often degrade the service lives of the pressurized parts, especially pressurized parts having thick walls. The thermal stresses experienced by the pressurized parts are often greatest during the transition of a combined cycle power generation plant from simple-cycle mode to combined-cycle mode and, in particular, when the HRSG has been offline for an extended period of time and the pressurized parts have cooled to ambient temperature.
Therefore, in order to reduce thermal stresses during transition from simple-cycle mode to combined-cycle mode, some combined cycle power generation plants temper the pressurized parts by gradually introducing the flue gas into the HRSG. Gradually introducing the flue gas into the HRSG, however, delays such combined cycle power generation plants in reaching full operational capacity. Moreover, the introduced flue gas typically has a temperature well above the pressurized parts, thus gradually introducing the flue gas into the HRSG may still induce thermal stress in the pressurized parts.
In view of the above, what is needed is a system and method for preheating a heat recovery steam generator.